msaa/formats: Adjust tolerance appropriately for floating-point formats.

When setting up a floating-point format, the GL implementation
responds to queries GL_ATTACHMENT_{RED,GREEN,BLUE,ALPHA}_SIZE with the
total number of bits in the floating-point value, regardless of
whether those bits are used for sign, exponent, or mantissa.  However,
for the purpose of setting a reasonable tolerance for pass/fail
comparisons, we really should only account for mantissa bits.

This patch modifies the MSAA "formats" test to detect when the buffer
format is GL_FLOAT, and if so, convert the channel size into the
number of mantissa bits, so that we don't test floating-point formats
to an unreasonably tight tolerance.
msaa/formats: Add floating point format tests.

The MSAA "formats" test now properly tests floating point formats.
This patch updates all.tests to cause those formats to be tested.
msaa/formats: Fix visualization of RED and RG formats

The switch statement in visualize_image() wasn't properly handling
formats whose base internal format is GL_RED or GL_RG.  As a result,
tests of these formats showed up as a black screen.  This patch addes
switch cases to handle the formats properly.
msaa/formats: Add ARB_texture_rg tests.

The MSAA "formats" test now properly tests RED and RG format
framebuffers.  This patch updates all.tests to cause those formats to
be tested.
msaa: Verify accuracy of sRGB MSAA resolves.

From the GL spec, version 4.2, section 4.1.11 (Additional Multisample
Fragment Operations):

    If a framebuffer object is not bound, after all operations have
    been completed on the multisample buffer, the sample values for
    each color in the multisample buffer are combined to produce a
    single color value, and that value is written into the
    corresponding color buffers selected by DrawBuffer or
    DrawBuffers. An implementation may defer the writing of the color
    buffers until a later time, but the state of the framebuffer must
    behave as if the color buffers were updated as each fragment was
    processed. The method of combination is not specified. If the
    framebuffer contains sRGB values, then it is recommended that the
    an average of sample values is computed in a linearized space, as
    for blending (see section 4.1.7).

This patch adds a new "srgb" mode to the MSAA accuracy test, which
verifies that the formula used by the implementation to blend sRGB
samples matches the GL spec's recommendations.  When an "srgb"
accuracy test is requested, the test is modified to do the following
things:

- Render using a buffer format of SRGB8_ALPHA8 instead of RGBA.

- When manually downsampling the reference image, enable
  FRAMEBUFFER_SRGB, so that the data output by the manual downsampler
  gets converted from linear color space into sRGB color space.  This
  ensures that the reference image is generated according to the
  spec's recommendations.

- Convert pixels from sRGB color space to linear color space before
  comparing the test image and the reference image.  This ensures that
  the RMS error is computed in linear color space, so that the RMS
  error computed for sRGB mode is comparable with the RMS error
  computed for linear RGBA color.