[rbp] Make gui limit default to 720 when memory is limited

On a 256M Pi, it's safest to default to 720p gui as memory is quite limited.
Merge pull request #5094 from popcornmix/limitres

[rbp] Make gui limit default to 720 when memory is limited
Move the reference-counting of Begin and End calls from DX and GL source files into GUIFontTTF.cpp.
Convert CGUIFontTTFBase::m_vertex to be managed as a std::vector.

Also retired CGUIFontTTFBase::m_vertex_count and
CGUIFontTTFBase::m_vertex_size because these can be derived from vector
member functions.
CGUIFontTTFBase::RenderCharacter can now append to arbitrary vectors of vertices rather than only CGUIFontTTFBase::m_vertex
Add a cache of font glyph bounding box vertices.

This is implemented as a template because ultimately we will key on different
parameters and store values of different types, depending upon whether we
have a GLES or non-GLES backend, and for GLES, whether or not the currently
applicable transformation matrices permit the use of hardware clipping.
Lay the groundwork for hardware clipping.

For glScissor() to replace CGraphicContext::ClipRect, a necessary condition
is that no shear or rotation is introduced between the coordinate systems
they use; this depends upon the settings of the GUI matrix m_finalTransform
as well as the OpenGL model-view and projection matrices. These all remain
unchanged between paired calls of CGUIShader::OnEnabled and
CGUIShader::OnDisabled, so we scan the matrices in CGUIShader::OnEnabled to
see whether hardware clipping is possible.

Then, in CGUIFontTTFBase::RenderCharacter, we don't apply software clipping
in such cases. However, because vertices arising from multiple
CGUIFontTTFBase::DrawTextInternal calls (each of which often uses a different
clip rectangle) get lumped into the same vector, which only at the end is
passed to OpenGL for rendering, we need to wait a few commits before we can
actually apply hardware clipping. In the meantime, expect to see rendering
errors.
Increase font cache hit rate by keying on the fractional part of m_originX and m_originY *after* they have been through the graphics context's transformation matrix, plus the scale/rotation elements of the matrix, rather than the origin in the original frame of reference plus the complete transformation matrix. All vertices of individual glyph bounding boxes are a constant offset from this position, and when the fractional part of the translation is a match, the rounding of each vertex will be in the same direction; this permits us to calculate the desired vertices from the cached ones simply by adding the integer parts of the translations with no additional rounding steps.

Note that this requires that software clipping is *not* performed.
Rewrite of scrolling text code.

No longer shuffles the string round to minimise the number of characters
before the clipping rectangle; this doesn't save much on rendering time but
does harm the effectiveness of the cache. Now uses a pixel offset into the
string rather than a character + pixel offset, and plots the entire string
every time (execpt when the wrap point is visible, in which case the entire
string is plotted twice).

It also makes motion smoother, because (possibly unintentionally) the
previous code preferred to align the scroll offset with character boundaries.
This would lead to uneven changes of position, especially when the width of
the character currently being scrolled off the edge was only slightly more
than an integral multiple of the scroll increment.
Move the application of the translation offsets into the GLES code.

Still all pure software at this stage. Main change is in the data types at
the interface between CGUIFontTTFBase and CGUIFontTTFGL. The old way
(array of vertices in m_vertex) are retained in addition, for the sake`of
cases that need to use software clipping on GLES, as well as for DX and GL
support where the new scheme is not (yet?) used.
Rather than applying the translation offsets to the vertices, now applies them to the model view matrix from the top of the matrix stack and pushes it over to OpenGL. The vertices themselves are still all held client-side.
Enable hardware clipping.
Move the vertex data across to a vertex buffer object just prior to drawing.
Move vertex data into an OpenGL VBO when the font cache entry is populated.

The font cache now stores the "name" (handle) of the VBO, rather than a vector
of vertices.
Switch from glDrawArrays() to glDrawElements().

This involves setting up a static VBO containing the indexes necessary to
convert from quads to triangles on the fly in the GPU.
[rbp] Don't override dvdplayer with omxplayer.

Using dvdplayer can be useful on the Pi. We can actually play sd (up to 640x480 MPEG-4 video) video in real time.
This is useful for codec variants like DivX3 which we don't currently play.

This may expose bugs where dvdplayer is incorrectly used as the default player which will need to be fixed
[players] Use default players rather than hard coded DVDPlayer/PAPlayer
[rbp] Don't force dvdplayer for airplay
[rbp] Give plugins omxplayer when they request dvdplayer on pi
[rbp] Allow ALSA to be chosen in addition to Pi sink

Needs --enable-alsa in ./configure step and alsa support on platform