In its current state, ScummVM seems to invoke these methods just after
destroying the current GL context. According to the OpenGL spec:
"Issuing GL commands when the program does not have a current
context results in undefined behavior, up to and including program
termination."
Our old behavior was to deref a `nullptr`, which isn't that great. For
now, protect these two methods. If other ports seem to misbehave as
well, we can always expand the check to other methods.
This is based mostly on Fabian "ryg" Giesen's 2011 blog series
"A trip through the Graphics Pipeline" and Scratchapixel's
"Rasterization: a Practical Implementation".
The rasterizer processes triangles in grid aligned 16x16 pixel blocks,
calculates barycentric coordinates and edge derivatives and interpolates
bilinearly across each block.
This will theoretically allow for better utilization of modern processor
features such as SMT and SIMD, as opposed to a classic scanline based
triangle rasterizer.
This serves as a starting point to get something on the screen.
In the future we might look into properly pipelining the main loop to
make the rasterizer more flexible, enabling us to enable/disable
certain features at the block rather than the pixel level.
This currently (obviously) doesn't support any actual 3D hardware,
hence all calls are done via software rendering.
Note that any modern constructs such as shaders are unsupported,
as this driver only implements Fixed Function Pipeline functionality.
The library is split into a base GLContext interface and a software
based renderer implementation of said interface. The global glXXX
functions serve as an OpenGL compatible c-style interface to the
currently bound context instance.
Co-authored-by: Stephan Unverwerth <s.unverwerth@gmx.de>
